1. Field of the Invention
The present invention relates generally to the field of medical devices, and more particularly, to the encapsulation of stents.
2. Description of Related Art
Stents and similar endoluminal devices are currently used by medical practitioners to treat portions of the vascular system that become so narrowed (stenosed) that blood flow is restricted. Such narrowing (stenosis) occurs, for example, as a result of the disease process known as arteriosclerosis. Angioplasty of a coronary artery to correct arteriosclerosis may stimulate excess tissue proliferation, which then blocks (restenosis) the newly reopened vessel. While stents are most often used to xe2x80x9cprop openxe2x80x9d blood vessels, they can also be used to reinforce collapsed or narrowed tubular structures in the respiratory system, the reproductive system, biliary ducts or any other tubular body structure. However, stents are generally mesh-like so that endothelial and other cells can grow through the openings resulting in restenosis of the vessel.
Polytetrafluoroethylene (PTFE) has proven unusually advantageous as a material from which to fabricate blood vessel grafts or prostheses, tubular structures that can be used to replace damaged or diseased vessels. This is partially because PTFE is extremely biocompatible causing little or no immunogenic reaction when placed within the human body. This is also because in its preferred form, expanded PTFE (ePTFE), the material is light and porous and is readily colonized by living cells so that it becomes a permanent part of the body. The process of making ePTFE of vascular graft grade is well known to one of ordinary skill in the art. Suffice it to say that the critical step in this process is the expansion of PTFE into ePTFE. This expansion represents a controlled longitudinal stretching in which the PTFE is stretched to several hundred percent of its original length.
If stents could be enclosed in ePTFE, cellular infiltration could be prevented, hopefully preventing restenosis. Early attempts to produce a stent enshrouded with ePTFE focused around use of adhesives or physical attachment such as suturing. However, such methods are far from ideal and suturing, in particular, is very labor intensive. More recently methods have been developed for encapsulating a stent between two tubular ePTFE members whereby the ePTFE of one-member touches and bonds with the ePTFE of the other member through the mesh opening in the stent. Unfortunately, such a monolithically encapsulated stent tends to be rather inflexible. In particular, radial expansion of the stent may stress and tear the ePTFE cover. Therefore, there is a need for a stent that is encapsulated to provide a smooth inner surface for the flow of blood and yet still allows expansion of the stent without tearing or delaminating, providing a relatively flexible device.
The present invention is directed to partially encapsulating stents wherein flexibility of the stent is retained, despite encapsulation. This can be done by placing a plurality of longitudinal strips over the stent or series of stents rings made of ePTFE and/or placing a plurality of circumferential ePTFE bands over the stent(s).
It is an object of this invention to provide a stent device that has improved flexibility, yet maintains its shape upon expanding or contracting.
It is also an object of this invention to provide a stent encapsulated to prevent cellular infiltration, wherein portions of the stent can move during radial expansion without stressing or tearing the encapsulating material.
These and additional objects are accomplished by embedding or encapsulating only a portion of the stent. In this way, the unencapsulated portion of the stent is free to move during expansion without compromising the ePTFE covering. The most straightforward way of achieving partial encapsulation is to place the stent(s) over an inner ePTFE tubular member (e.g., supported on a mandrel) and then to cover the outer surface of the stent(s) with a series of spaced apart longitudinal ePTFE strips, which are then laminated to the inner ePTFE to capture the stent. These strips (e.g., cut from an extension of the inner ePTFE tube) can be woven about the stent(s) and later laminated into position to provide an anti-compression function as well as overall structural stability. Beside strips of ePTFE it is also possible to use circumferential ePTFE bands to further or alternatively capture the stent(s). By selecting the size and position of the bands it is possible to leave critical parts of the stent unencapsulated to facilitate flexibility and expansion. Although a single stent can be used, these approaches lend themselves to use of a plurality of individual ring stents spaced apart along the inner ePTFE tube.
In the present invention, individual ring stents are partially encapsulated using the procedure outlined above. Preferably, ring stents of zigzag sinusoidal structure are placed xe2x80x9cin phasexe2x80x9d on the outside surface of a tubular ePTFE graft supported by a mandrel. Separate bands of ePTFE are placed over the stent rings, so that some portion of the stent rings is covered. In addition, longitudinal strips of ePTFE can be woven (e.g., over and under) about the ring stents, either before or after the bands are applied. The resulting structure is then subjected to heat and pressure so that the regions of ePTFE become laminated or fused together. In addition, the ends of the stent can be completely encapsulated, by known methods, to stabilize the overall structure.